Microphones have been widely used e.g. in sound recording, in applications of speech and music recording, in sound level measurements, and in environmental noise level measurements.
In sound wave measurements, e.g. sound recording, it is important to measure the sound waves with high sensitivity and in great detail i.e. with a large dynamic range and linear response. Also, it is important that the response of the microphone does not change in temperature and humidity variations.
A typical microphone is a transducer, which converts acoustic energy to electrical energy. Typically the fluctuating acoustic energy vibrates a diaphragm and the displacement of the diaphragm is converted to an electrical signal proportional to the acoustic energy. Various types of microphones are known, which vary in the accuracy and sensitivity of detecting the original acoustic energy.
Typically high quality audio microphones use a capacitive measurement principle. The drawback of the capacitive measurement principle is that high sensitivity is gained only by bringing the back plate (electrode) close to the diaphragm (electrode). This creates damping of the system and lowers the Q-value of the diaphragm increasing the self noise, which is created by the Brownian motion. In addition the existence of the back plate creates extra non-linearity.
Furthermore, a typical drawback of capacitive microphones is that the dynamic range is related to the sensitivity. For example capacitive microphones with a wide dynamic range have poor sensitivity and microphones with better sensitivity usually have a narrow dynamic range.
In order to achieve a microphone with high sensitivity and a wide dynamic range simultaneously the displacement of the sensor should be measured optically without disturbing the sensor movement and directly in a digital form.
Patent publication GB 1267632 discloses a digital optical microphone, particularly for a telephone handset, that includes an interferometer consisting of a two-prism block and a mirror attached to the microphone diaphragm. Infrared radiation from a diode is reflected off the moving mirror and the back face of the block, interferes and is detected by photodiodes. The two photoelectric signals are in phase quadrature due to the different thicknesses of reflecting coating on the mirror, which reflect light to the photodiodes respectively. The two signals may be delta-modulated by a logic circuit, which may additionally include a winding, providing a biasing force on the diaphragm, which receives an integrated value of the two photoelectric signals.
With the microphone according to GB 1267632 diaphragm displacements no smaller than λ/4, where λ is the wave length of the light source in the interferometer, can be measured, which is not a good accuracy. The focuses of the light beams are in infinity in relation to the mirrors and therefore the stability of the system is easily disturbed by inclination of the diaphragm and the mirror attached to it and the reference mirror.